By the Numbers: An Epidemic of Redundant Reviews

Little value seen for most meta-analyses or systematic reviews

The number of articles in PubMed appearing each year has been rising, of course, but publication of systematic reviews and meta-analyses, "often produced either by industry employees or by authors with industry ties," has increased even more -- like, 10 times faster since 1986, writes John Ioannidis, MD, of Stanford University, in an analysis published in the Milbank Quarterly.

The growth in such studies alone wouldn't be so bad, Ioannidis wrote, if they were approached as "preemptive" collaborations "in consortia with embedded replication across teams and joint analyses." Instead, he wrote, many are not useful, instead serving only as marketing ploys or to pad authors' CVs.

The largest producer of such works have affiliations with China, particularly for genetic association analyses. "Perhaps the reasons for this are that meta-analyses can be done with little or no money," Ioannidis wrote.

The paper goes further, attempting to quantify what proportion of these derivative works are actually useful. A large percentage were deemed "misleading" due to reliance on few genes or variants; about 20%, he estimated based on other research, went unpublished. A similar number were "decent but not useful," in that their ultimate conclusions were that the available evidence was weak or insufficient, meaning that the analysis couldn't help inform clinical decision-making.

These verdicts, and others summarized in the chart below, were made off-the-cuff (the precise-looking percentages notwithstanding; Ioannidis didn't claim to have studied all of the nearly 60,000 meta-analyses published since 1986). But anyone who has read more than a few meta-analyses is likely to find them credible.

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